Fuel cells are often described as continuously operating batteries or as electrochemical engines. Fuel cells utilize an external supply of fuel and oxygen (or air) and produce power continuously, as long as the fuel and oxygen supply is maintained.
The most classic fuel cell is the H2/O2 fuel cell of the direct or indirect type, wherein hydrogen is oxidized to form H3O+ at the anode and oxygen is reduced to water at the cathode. In the direct type, hydrogen and oxygen are used as such, the fuel being produced in independent installations. The indirect type employs a hydrogen-generating unit, which can use as raw material a wide variety of fuels.
Another type of fuel cell is the organic fuel cell. In a direct oxidation cell an aqueous solution of an organic fuel such as methanol, formaldehyde or formic acid, is directly fed into the fuel cell without any previous chemical modification, where the fuel is oxidized at the anode, and oxygen is reduced to water at the cathode.
A major distinguishing characteristic of different fuel cells is in the electrolyte used. NASA's Jet Prepulsion Laboratory (JPL) developed a direct liquid-feed cell using a solid membrane electrolyte. A detailed description of JPL's fuel cells can be found, for example, in U.S. Pat. Nos. 5,599,638 and 5,773,162. These fuel cells operate without any acid electrolyte and comprise solid electrolyte membranes fabricated from proton-exchange materials, especially Nafion™ (manufactured by DuPont). When methanol is used as the fuel, the electro-oxidation of methanol at the anode can be represented by:CH3OH+H2O→CO2+6H++6e,and the electro-reduction of oxygen at the cathode can be represented by:O2+4H++4e→2H2O.
Protons generated at the anode are transported directly across the electrolyte membrane to the cathode. A flow of current is sustained by a flow of ions through the cell and electrons through the external load.